A Xray detector, based on the CsI(Tl) scintillating crystal and areaarray CCD, using fiber and fiber optic plate to couple and transmit light with fanbeam linear array scanning, has been put forth. The luminescent efficiency and Xray high-resolution detection are directly influenced by the dimension of CsI(Tl) scintillating crystal, and Monte Carlo method is adopted to simulate the influences, including the relations between the energy of Xray, the distance between Xray source and the scintillating crystal, and the thickness of CsI(Tl) crystal and the Xray energy distribution, and then the whole energy peak efficiency and the transition efficiency. The results show that while the Xray energy is of 120～450 keV, and the thickness of CsI(Tl) crystal is 0.3～1.5 cm, the whole energy peak is within the range of 31.34%～96.74%, and the transition efficiency is within the range of 12.8%～97.43%. So, the Xray energy and the dimension of CsI(Tl) scintillating crystal are the most important parameters determining the Xray high resolution detection. It has certain referenced values to optimize the dimension design of CsI(Tl) scintillating crystal for Xray high resolution detection.